In the related art, as a bonding apparatus configured to connect a pad on an IC chip and an external lead, a wire bonding apparatus as illustrated in FIG. 7 is known.
FIG. 7 is a drawing illustrating a configuration of a wire bonding apparatus of the related art, FIG. 8 is a drawing illustrating a tool height and a search level of the wire bonding apparatus of the related art, FIG. 9(a) illustrates a speed waveform when a bonding tool (capillary) of the wire bonding apparatus of the related art is lowered, and (b) is a drawing illustrating a trajectory of the bonding tool when being lowered.
As illustrated in FIG. 7, a wire bonding apparatus 50 of the related art includes an ultrasonic transducer (not illustrated) and composed of a bonding head 51 including an ultrasonic horn 53 having a capillary 6 as a bonding tool 6 mounted at a distal end thereof, a bonding arm 52 having at one distal end thereof the ultrasonic horn 53 and being coupled at the other end thereof with a supporting shaft 9, an encoder 8 as position detecting means configured to detect the position of the capillary 6 mounted at the distal end of the bonding arm 52, and a linear motor 10 configured to drive the bonding arm 52 upward and downward about the supporting shaft 9, an XY stage 20 as positioning means on which the bonding head 51 is mounted and configured to move the same two dimensionally in an X direction and/or a Y direction and position the same, a bonding stage 22 on which a lead frame having an IC chip 40 or the like mounted thereon is mounted and configured to perform a bonding operation by the capillary 6, a controller unit 55 including a micro processor, and a drive unit 57 configured to emit a drive signal to the bonding head 51 and the XY stage 20 according to a command signal from the controller unit 55.
Also, the wire bonding apparatus 50 also includes a camera (not illustrated) as image pickup means configured to pick up an image of a surface of a bonded part before starting bonding to detect positional displacement of the IC chip 40 and a lead 41 mounted on the bonding head 51.
In the wire bonding apparatus 50, it is necessary to set various conditions, that is, set bonding parameters before performing a series of bonding operations.
The bonding parameters include a tool height, a search level, a search speed, and so forth. Referring now to FIG. 8, these bonding parameters will be described below.
As illustrated in FIG. 8, the tool height (also referred to as “tool height” means a distance from an origin position (O in FIG. 8) of the capillary 6 mounted at the distal end of the ultrasonic horn 53 to a surface of the pad on the IC chip 40 as a first bonding point (La in FIG. 8) and a distance to a surface of the lead 41 as a second bonding point (Lb in FIG. 8). The origin position of the capillary 6 is set in advance. The setting of the tool heights is performed by moving the capillary 6 from an origin position O of the capillary 6 set in advance at a low speed, and obtaining the distance until the distal end of the capillary 6 reaches the bonding surface, that is, the pad of the IC chip 40 or the lead 41 by counting outputs from the encoder 8 as position detecting means configured to detect the position of the capillary 6.
Also, as illustrated in FIGS. 9(a) and (b), a search level S means a height of the capillary 6 when the speed of downward movement of the capillary 6 is changed from a high speed to a low speed. The capillary 6 moves downward at a high speed, reduces the speed at a point before the search level S, and is moved downward at a search speed (Vs in FIG. 9(a)) which is a constant low speed lower than the search level S, and hence a ball engaged at a distal end of the capillary 6 comes into abutment with the pad at the first bonding point. On the other hand, at the second bonding point, the wire lead from the distal end of the capillary 6 comes into abutment with the surface of the lead 41.
As illustrated in FIG. 8, setting of the search levels of the IC chip 40 and the lead 41 on the lead frame L/F is different. In other words, as illustrated in FIG. 8, a height (h1 in FIG. 8) from the surface of the IC chip 40 to a search level (S1 in FIG. 8) is set on the side of the IC chip 40, and a height (h2 in FIG. 8) from the surface of the lead 41 to a search level (S2 in FIG. 8) is set on the lead 41 side. For reference, the search levels on the IC chip 40 side and on the lead 41 side may be the same value.
The reason why the setting of the search level is performed is as follows. In other words, the IC chip 40 mounted on the lead frame L/F or the like is joined with a conductive joint member such as resin, solder, gold, and the like, and hence mounting heights of the IC chips 40 vary from one IC chip 40 to another due to variations in thickness of the joint members. Also, since the joint members tend to have an uneven thickness, the IC chip 40 may be mounted on a tilt in the height direction. On the other hand, as regards the leads as well, the heights of the surfaces of the leads due to variation in thickness of the lead members and warping or the like of the leads.
Therefore, when there are variations in height of the IC chip or the leads, the amount of deformation of the ball may vary because of increase in impact force with respect to the pad upon abutment due to the variation in height or the like, or timing of application of the bonding load and an ultrasonic power may become unequal, whereby the reliability of the bonding may be lowered due to instability of the diameter of ball pressure bonding, joint strength, and the like.
Therefore, the heights of the IC chips and the leads, which are bonded parts, may vary, and hence the heights of the tool vary from one IC chip to another and from one lead to another. Therefore, in order to compensate such variations, the pads and the leads are brought into abutment with each other at a low constant speed. In this manner, the search level S as the bonding parameter needs to be set with due consideration of the variation in height of the IC chips and the variation in height of the lead or the like.
Also, the search speed Vs means a speed of downward movement of the capillary 6 at a level from the search level downward, that is, from a height where the speed of downward movement of the capillary 6 is changed from the high speed to the low speed downward. The search speed Vs is for reducing the amount of initial deformation of the ball due to the impact of the ball upon abutment with respect to the pad.
Subsequently, a bonding process of the wire bonding apparatus of the related art will be described. The wire bonding is performed by moving the capillary 6 downward at a high speed, reducing the speed at a point before the search level S1 illustrated in FIG. 8, moving the same downward from the search level S1 downward at a constant search speed Vs, and presses the ball at the distal end of the capillary 6 against the pad. After the encoder 9 as position detecting means has detected the fact that the ball comes into abutment with the pad, a predetermined bonding load is applied and, simultaneously, an ultrasonic vibration is applied to the distal end of the capillary 6 via the ultrasonic horn 53, whereby the ball is connected to the pad.
Subsequently, the capillary 6 is moved upward according to a predetermined loop control, and is moved in the direction of the lead, which corresponds to the second bonding point. The capillary 6 is lowered at a high speed, is lowered at the search speed Vs from the search level S2 illustrated in FIG. 8, and presses the wire at the distal end of the capillary 6 against the lead.
After the encoder 9 as position detecting means has detected the fact that the wire comes into abutment with the lead, a predetermined bonding load is applied and, simultaneously, an ultrasonic vibration is applied to the distal end of the capillary 6 via the ultrasonic horn 53, whereby the wire is connected to the ball.
Subsequently, the capillary 6 is moved upward to cause the wire to protrude from the distal end of the capillary 6 to a predetermined length, a wire cut clamp (not illustrated) is closed at a preset predetermined rising position of the capillary 6, the wire on the lead is cut, and the wire is extended from the distal end of the capillary 6 by a predetermined length. Subsequently, high voltage is applied between a spark rod (not illustrated) and a wire to form a ball at the distal end of the wire.
As described above, the wire bonding apparatus 50 of the related art sets the search level S in order to compensate the variation in height of the IC chips and the variation in height of the leads, reduces the speed before the search level S, and is further lowered at the sufficiently reduced search speed Vs, then, the fact that the ball or the wire at the distal end of the capillary comes into abutment with the surface of the bonding point is detected by the position detecting means, and then bonding is performed.
However, determination of stop of the bonding arm of the position detecting means is determined later than the actual timing of abutment because determination of abutment with the bonding point is performed when a value of difference between a command speed signal for driving the bonding arm and a feedback speed signal from the encoder reaches or exceeds a preset predetermined value.
In other words, in the wire bonding apparatus 50 of the related art, since the detection is performed after the capillary comes into abutment actually with the pad (or the lead) of the IC chip by the encoder as the position detecting means at the sufficiently reduced search speed Vs, the movement time of the ball or the wire at the distal end of the capillary until coming into abutment with the pad from the search level S is several tens of msec, and there is a problem that the position of abutment is detected several msec behind the actual abutment.
When the distance from the search level to the bonding point is increased or the detection of position by the position detecting means is delayed, time is increased, and the proportion of such temporal delay and the increased time with respect to the entire bonding time is increased, and hence there arises a problem that reduction of the bonding time is difficult.
Therefore, Patent Literature 1 discloses a wire bonding apparatus including a laser displacement gauge configured to detect the height of a position where the wire of the semiconductor chip is bonded in a contactless manner and a control apparatus configured to control the driving of the bonding tool toward the target of driving on the basis of a detection signal from the laser displacement gauge and bond the ball to the electrode of the IC chip, and configured to be capable of bonding without excessively crashing the ball formed on the wire against the work.
Also, Patent Literature 2 discloses a wire bonding apparatus configured to reduce the bonding time by irradiating the semiconductor chip, a substrate, and a lead frame as bonding surfaces with a laser beam, calculating the amount of deviation from a spot position stored in advance, and calculating the height of the actual bonding surface before bonding by obtaining the difference in a height direction from the amount of deviation.